This invention is directed to a flowable flax bast fiber and flax shive blend which may be used as a reinforcing agent for thermoplastic resins.
A number of different materials, such as organic and inorganic fibers, have been used to make thermoplastic composite reinforcements. Inorganic fibers include glass, carbon, metals or metal alloys, such as steel or aluminum, and stone. Organic fibers include aramid, nylon, polyester, polypropylene, polyethylene and natural fibers, such as cotton and wood.
Traditionally, milled fiberglass has been the most popular material for reinforcement of thermoplastic compounds. Fiberglass has a unique combination of versatility and strength that makes this reinforcement a material of choice for more than 50% of all composite articles manufactured in the year 2000, and the most popular choice for thermoplastic reinforcement as well. Synthetic organic fibers (nylon, polypropylene, aramid, etc.) are used occasionally and for specialty applications only. Despite their good availability, man-made fibers have significant disadvantages, including high prices tied to crude oil prices. All of these materials pressure the environment because they are not necessarily renewable, are not biodegradeable and generate significant Green House Gas emissions upon manufacture and/or destruction. Key disadvantages of fiberglass also include the worker-unfriendly nature of the material (fiberglass is an irritant), its fragility which makes it difficult to process; and finally, its density (natural fibers have specific density that is 40% less than density of fiberglass).
Because flax bast fibers are difficult to feed into resin molding equipment, they have not been used for thermoplastic reinforcing agents. Although the potential of using flax fibers in plastic applications as a substitute for synthetic fibers such as glass, carbon, nylon, polyester, etc. has been recognized, for example, GB 2090849 describes the incorporation of flax bast fibers into a thermoplastic resin blend. The use of flax bast fibers for reinforcing resins results in processing problems, as the flax bast fibers tend to xe2x80x9cball upxe2x80x9d during processing. Thermoplastic resins blended with specific amounts of flax shives and bast fibers are not described. Further, purposeful attempts have been made to exclude flax shive from flax fiber preparations used for reinforcing composites. For example, Mieck et al. (Polymer Composites, December 1996, Vol. 17, No. 6), suggest that for better composite characteristics, the fibers should be free from shives. Flax bast fibers have very low-bulk density which tends to cause the aforedescribed clumping or balling. At the same time, almost all inorganic fibers have a very good feeding properties due to high-bulk density. However, this feeding advantage of inorganic fibers creates a problem for the composite. It makes composites heavy.
U.S. Pat. Nos. 6,133,348 and 6,114,416 to Kolla et al. describe a shive/bast fiber blend used as a resin reinforcing agent where the blend has 30 weight percent bast fibers as a maximum. If the blend described in these patents has more than 10 weight percent bast fibers, without using shives and bast fibers as described herein, feeding problems will be incurred because the bast fibers will ball and clump. Neither the ""348 patent or the ""416 patent suggest a flax bast fiber/shive blend with the aspect distribution and/or particle size distribution described herein.
It also has been known that fibers could be chopped or reduced in their length to improve the reinforcement feeding properties. This approach is widely used with inorganic fibers. Milled glass fibers are glass fibers processed by a hammer mill into lengths of {fraction (1/32)}xe2x80x3 to {fraction (1/81)}xe2x80x3. There are many brands of milled inorganic fibers (e.g., West System {fraction (1/32)}xe2x80x3 Milled Glass Fiber), which found broad application as reinforcement for thermoplastic compounds.
It is an object of the invention to provide a blend of flax bast fibers and flax shives which are flowable and which will not clog or clump in thermoplastic resin forming equipment.
This and other objects of the invention will become apparent with reference to the specification.
This invention relates to free flowing fiber reinforcing material which includes a blend of flax bast fibers and flax shives with a specific distribution of particle sizes and aspect ratios, thermoplastic pellets which include such reinforcing fiber blend, thermoplastic composites which include such reinforcing fiber blend and method for making such pellets and composites. The flax bast fibers and flax shives each have particle size ranges in amounts which are sufficiently similar and short which makes the blend free flowing and feedable into equipment which receives and shapes thermoplastic resins without the addition of other surface treating agents for the flax bast fibers. The particle sizes of the bast fiber are sufficiently long to make the bast fiber/shive blend an effective reinforcing agent for thermoplastic resins. Moreover, for a given particle size, the shives have a smaller average aspect ratio value than the plant bast fibers. The similarity of the amounts of flax shives and bast fibers in a given particle size range, and the smaller aspect ratio of the shives compared to the aspect ratio of the bast fiber all are effective for making the reinforcing blend freely flowable and feedable into resin processing equipment. Once introduced into such resin processing equipment, the blend can be easily compounded with a thermoplastic resin, which is softened by heating, and the resulting flowable mixture can be thermoformed into desired bast fiber/shive reinforced composite shape.
The resulting fiber reinforced composites are relatively light-weight yet strong and flexible structures well-suited for diverse applications. In one aspect, solid flowable pellets are made as an intermediate product which is a mixture of the flax bast fiber/flax shive blend and thermoplastic resin to provide an intermediate composite product. The fiber reinforced resin pellets can be used subsequently as a convenient ready-to-use feed for an injection mold or extruder.
As noted above, heretofore flax bast fibers have been known to clump or xe2x80x9cball upxe2x80x9d and obstruct their feeding into and or within passageways in resin processing equipment. Surprisingly, the blend of the invention which has at least about 15 weight percent flax bast fibers can significantly minimize or eliminate such clumping, and, in general, the blend should have at least about 30 weight percent bast fiber to further enhance the reinforcing properties of the blend. The blending of the flax bast fibers and flax shives can be accomplished before or during compounding with the thermoplastic matrix resin in resin processing equipment. When preblended, a free flowing form of flax fibers and shives is provided suitable for clog-free feeding into resin processing equipment. Once introduced into the resin processing equipment, the flax bast fibers and shives in the blend continue to interact in a manner that effectively inhibits and curbs clumping problems arising from the bast fibers. These problems would occur if a natural ratio of flax bast fiber and flax shive were used during compounding and movement of the fiber/flax shive/resin blend within the equipment.
While not desiring to be bound to any theory, it is believed that the flax shives surround exterior portions of the low bulk density flax bast fibers when blended according to the particle size and aspect ratio distribution criteria of this invention. This sufficiently inhibits the bast fibers from physically bridging or hooking together to otherwise form undesired clumps or agglomerates of bast fibers. It is believed that the flax shives serve to xe2x80x9clubricatexe2x80x9d the flax bast fibers to effectively suspend them as a shive/bast fiber dispersion and keep the bast fibers from mechanically entangling with one another. As a consequence, a relatively freely-flowable blend of flax shives and bast fibers is achieved that permits substantially uniform feeding into and through constrictions and feeding inlets of resin processing equipment.
In one aspect, the flax bast fiber portion comprises from about 15 to about 70 weight percent of the blend and the flax shive portion comprises from about 85 to about 30 weight percent of the blend. The flax bast fibers of the bast portion and the shives of the shive portion have a plurality of particle size ranges and aspect ratio ranges which effect the flowability of the blend and make it more flowable than a comparative blend that comprises about 70 weight percent shives and about 30 weight percent bast fiber without any adjustment of the distribution of particle sizes or aspect ratios from that which is naturally supplied by the flax plant when it is decorticated. A blend of about 70 weight percent shives and 30 weight percent bast fibers is that ratio of shives to fiber in a flax plant and which would be obtained from flax upon decorticating (i.e., stripping and cleaning) a flax plant as would be described in U.S. Pat. Nos. 6,079,647; 5,906,030 and 5,720,083 which are incorporated herein and without milling and/or particle separation of the shive and bast particles subsequent to decortication. The particle size distribution and aspect ratios as described herein are obtained by screening and aspiration equipment as is known in the art down stream of the methods and equipment described in the aforedescribed patents. The distribution of respective particle sizes of bast fibers and shives and aspect ratios of the flax bast fiber portion and the flax shive portion are selected and blended to effect the above-discussed unique lubricating effect on the flax bast fibers and prevent them from clumping or at least reduce the clumping as compared to a blend having a ratio of shives to bast fibers of about 7 to about 3, where the blend has particle sizes and aspect ratios which have not been preselected to provide flowability in a thermoplastic resin. It has been discovered that a flax bast fiber portion can be provided that not only is conducive for making such a flowable fiber blend, but which also still possesses a particle size effective for increasing at least one of the tensile and flexural strength of the resin in which it is embedded, such as an increase of at least about 10%, when the flax bast fiber portion is loaded in the resin in an amount of 30 percent by weight. Thus, the flowability of the flax bast fibers is achieved without sacrificing or unduly diminishing the ultimate reinforcing properties or other positive attributes contributed by the flax bast fibers to thermoplastic composites in which they are embedded.
The natural flax fiber content in the fiber reinforced composites of this aspect of the invention effectively displaces more costly thermoplastic matrix resin content or thermoplastic fiber reinforcements in the composite. The natural plant fiber materials such as flax used in the invention are low cost, light-weight, available in many regions of the world and environmentally friendly materials. Moreover, a freely-flowable plastic matrix composition suited for handling in plastic processing equipment is provided without the need to resort to inorganic fibrous reinforcing materials, such as fiberglass, having disadvantages including increased weight and their irritant nature.